The present invention relates generally to the field of wireless telecommunications, and, more particularly, to methods and apparatuses for resource managements for devices operating in a multi-carrier system e.g. the LTE-advanced (Long Term Evolution) system.
The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the UMTS (Universal Mobile Telecommunication Service) system, and LTE is currently under discussion as a next generation mobile communication system of the UMTS system. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). Thus work is ongoing in 3GPP to specify an evolution to UTRAN, denoted E-UTRA, as part of the LTE effort. The first release of LTE, referred to as release-8 (Rel-8) can provide peak rates of 300 Mbps, a radio-network delay of e.g. 5 ms or less, a significant increase in spectrum efficiency and a network architecture designed to simplify network operation, reduce cost, etc. In order to support high data rates, LTE allows for a system bandwidth of up to 20 MHz. LTE is also able to operate in different frequency bands and can operate in at least frequency division duplex (FDD) and time division duplex (TDD). Other operation modes can also be used. It should be noted that OFDM (orthogonal frequency division multiplexing) is supported in LTE.
For the next generation mobile communications system e.g. IMT-advanced and/or LTE-advanced, which is an evolution of LTE, support for bandwidths of up to 100 MHz is being discussed. One issue with such large bandwidth is that it is challenging to find free 100 MHz of contiguous spectrum, due to that radio spectrum a limited resource.
It should be mentioned that LTE-advanced can be viewed as a future release, denoted release-10 (Rel-10) of the LTE standard and since it is an evolution of LTE, backward compatibility is important so that LTE-advanced can be deployed in spectrum already occupied by LTE (e.g. Rel-8). This means that for a LTE user equipment or a LTE terminal, a LTE-advanced capable network can appear as a LTE network.
As mentioned earlier, in LTE-advanced that can support 100 MHz of bandwidth. This can be performed by aggregating non-continuous spectrum, to create, from e.g. a baseband point of view, a larger system bandwidth. This is also known as carrier-aggregation, where multiple component carriers are aggregated to provide a larger bandwidth. FIG. 1 illustrates an example of carrier aggregation which is aggregation of multiple component carriers. Each component carrier can appear as an LTE carrier, while an LTE-advanced terminal (or UE) can exploit the total aggregated bandwidth. As shown in FIG. 1, each bandwidth of e.g. 20 MHZ represents one component carrier. The LTE-advanced system can therefore be viewed as a multi-carrier system. Aggregation of multiple component carriers (CC) allows large bandwidth for supporting data rates of 1 Gb/s or even above, which corresponds to the throughput requirement for the (international mobile telecommunications) IMT-advanced system. Furthermore, such a scenario makes it also possible to adapt the spectrum parts to the current situation and geographical position making such a solution very flexible. To an LTE Rel-8 UE each CC (e.g. of 20 Mhz bandwidth), as shown in FIG. 1, will appear as an LTE carrier, while an LTE-advanced UE can use all 5 CCs i.e. the total aggregated bandwidth (e.g. 100 MHz) such as the one shown in FIG. 1. Thus, a Rel-8 LTE can be viewed as a single carrier system whereas a LTE-advance (Rel-10) can be viewed as a multi-carrier system.
In LTE, a UE's first access to the system is performed by means of a so called random access (RA) procedure. The objectives of the RA procedure may include: initial access; connection re-establishment, handover; scheduling request (request for radio resources); timing synchronization, and the like. The radio network nodes generally control the behavior of the UE. As an example, uplink transmission parameters like frequency, timing and power are regulated via downlink control signalling from the radio base station (known in LTE as eNB or eNodeB) to the UE. For the uplink (UL) frequency and power estimate parameters, a UE can derive those parameters from one or several downlink (control) signals.
The RA procedure can be classified into a contention-based random access procedure and a contention-free (or non-contention-based) random access procedure. The RA procedure is disclosed in the 3GPP technical specifications 3GPP TS 36.321 entitled: “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) Medium Access Control (MAC) protocol specification (Release 8)”
As an example, for an initial access to the network, the RA procedure is contention-based in which case the UE follows a contention resolution procedure. A contention resolution allows a UE to determine whether or not resources granted by the network as a response to the RA are intended to the UE. Contention resolution is important because multiple UEs may attempt to access the system using the same common resource (e.g. physical random access channel (PRACH) and the same randomly selected preamble.
It should be noted that for the contention-based random access procedure, a plurality (or a set) of non-dedicated random access preambles are assigned per cell (i.e. to a eNB). This set is primarily used when there is UE-originated data and the UE has to establish a connection and an adequate uplink timing relation with the network through the RA procedure. When performing contention-based random access, the UE arbitrarily selects a preamble from the set as the non-dedicated random access preamble. This is known as UE initiated random access (supported in LTE). Thus for contention-based random access, the network (or the eNB) is not (immediately) aware of which UE selected which preamble. A drawback with this is that multiple UEs may in fact select the same preamble and they may attempt to access the network (or access the eNodeB) at the same time. Therefore the contention resolution mechanism is important.
Prior to accessing the single carrier system using the RA procedure, the UE needs the available set of PRACH resources for the transmission of the RA Preamble; the UE may acquire these parameters by reading the broadcasted system information of the cell or these parameters may be included in the message sent by the source eNB in case of handover.
For accessing the system when the UE is already known to the network, a contention-free RA is also possible. For performing contention-free random access, there is also defined a set of RA preambles assigned per cell (i.e. to a eNB). These preambles are known as dedicated RA preambles. The dedicated preamble is an example of a temporary unique identity to be used on the common resource, and is allocated to the UE prior to the access.
FIG. 2 illustrates a simplified flow diagram depicting steps used in a RA procedure in case of initial access (e.g. contention-based).
The contention-based RA procedure consists of four steps. In the first step, the UE transmits MSG1 which consists of a randomly selected preamble; the UE will later, on a resource calculated based in the PRACH resource used for the transmission of the preamble, monitor for a (RA) response (i.e. MSG2) from the network, which response includes the transmitted preamble.
In the second step, the UE receives and successfully decodes MSG2 containing a preamble that matches the one sent in the previous step. MSG2 includes a Temporary C-RNTI i.e. a temporary identity for the UE which purpose is to identify the UE when resolving contention and a grant for a dedicated transmission on UL-SCH (uplink shared channel).
In the third step, the UE transmits MSG3 which contains a MAC SDU filled with data from upper layers that triggered the initial access to the system. Once MSG3 is sent, the UE monitors the PDCCH (physical downlink control channel) for the Temporary C-RNTI received in the second step.
In the fourth step, the UE has successfully decoded the PDCCH for the Temporary C-RNTI; if the received MAC SDU contains a copy of the MAC SDU (media access control service data unit) transmitted in MSG3, the UE assumes that it won the contention and continues dedicated transmissions using the Temporary C-RNTI as its C-RNTI.
The RA procedure (contention based or not) can also be used when the UE is known from the network (i.e. when the UE has a valid C-RNTI) for the purpose of gaining access to uplink resources and/or uplink timing synchronization, in case the UE is not assigned dedicated resources on e.g. the PUCCH (the physical uplink control channel) for transmission of a scheduling request.
Furthermore, the RA procedure can be initiated by an PDCCH order triggered by the eNB. The eNB can send a PDCCH consistent with a PDCCH order masked with a C-RNTI and as the UE receives the PDCCH transmission, the UE initiates a RA procedure. The PDCCH order can indicate a RA preamble (or the identity of the preamble) and resource information i.e. PRACH information.
The RA procedure is also used when a UE performs a handover (HO) from a serving eNB to a target eNB. In LTE (Rel-8), the HO procedure is described in 3GPP TS 36.331 entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC)”;
In short, the source eNB starts by making a decision to handover the UE based on some criteria e.g. measurement report(s) received from the UE. Then the source eNB can prepare or setup a target eNB and further make additional preparations before transmitting a RRC (radio resource control) connection reconfiguration with mobility information message (aka HO command) towards the UE. The UE then detaches from the cell that is served by the source eNB and the UE synchronizes to the new cell served by the target eNB. Thereafter, the UE performs a RA in the new cell. The RA can be contention-free if the preamble was received in the HO command, else the previously described 4 steps for contention-based are used to perform the RA. Thereafter, the UE transmits RRC connection reconfiguration complete message. Note that additional steps are performed which are not explicitly described above.
Note also that at HO, it is possible for the target eNB to signal a dedicated preamble in the HO command to the UE which can be transmitted via the source eNB prior to the change of the serving cell. This can be performed to speed up the HO procedure by the UE in the target cell and to speed up network-initiated access by the UE in the serving cell.
As mentioned before, LTE (Rel-8) is a single carrier system in which a “cell” can correspond to only one component carrier (CC). The RA procedure, due to an ordered RA or due to a HO, is important for enabling a UE to successfully access resources and commence transmissions. Also mentioned earlier is that in LTE, a number of preambles can be reserved for dedicated use in a given cell (or in a given CC) and the dedicated preambles may be transmitted in the same PRACH resources as random preambles. Thus in single-carrier LTE, a UE can access PRACH resources in the cell (CC) for which the dedicated preamble is valid for the UE.
However, a multi-carrier system (e.g. LTE-advanced) can be defined using either a plurality of single carrier cells or as a single cell with a plurality of CCs (the latter will be assumed from this point on, however not limiting the applicability of the invention herein). In a multi-carrier system, to acquire system information, the UE may be required to first monitor the broadcasted system information in one of the carriers to determine the structure of the multi-carrier cell. The UE could then either perform random access immediately using the PRACH resources of this carrier (if allowed by the system configuration), or alternatively monitor system information on one or more of the other carriers to locate other allowed PRACH resources (if any). A carrier could also broadcast information on location of PRACH resources in other CCs, which would however represent some overhead to the system.
From the multi-carrier capable UE's perspective, the process of finding the first allowed and suitable PRACH resource and opportunity may represent considerable processing and additional latency in accessing the system and commencing transmissions. From the network's perspective, broadcasting in one or more CCs system information containing a description of the PRACH resources available in other carriers represents additional complexity and overhead. In addition, the network has little control over what resource the UE will use and thus it becomes a challenge to efficiently manage the system resource related to PRACH.
Therefore, because in a multi-carrier system several CCs are defined per cell, where PRACH resources may or may not be allocated and when allocated may be offset in time between each CC, a HO procedure and/or a PDCCH-ordered RA of a UE in such a multi-carrier cell can take long time compared to single-carrier system thereby introducing/increasing access delays or access latency e.g. HO latency and RA-ordered latency.